Such a composite copper foil is disclosed in U.S. Pat. No. 3,998,601. It comprises a thin copper foil-hereinafter called thin functional foil-mounted on a carrier foil, wherein the carrier foil is provided with an intermediate, very thin release layer which permits clean and easy separation of the thin functional foil. The carrier foil, when produced by conventional electrolytic techniques, will have a smooth cathode side (also known as shiny side) formed in contact with a rotating titanium cathode drum and an opposite, rough electrolyte side (also known as matte side). The surface of the thin functional foil conforms to that of the carrier foil, since the release layer is very thin. U.S. Pat. No. 3,998,601 further suggests that the thin functional foil should be deposited on the electrolyte side of the carrier foil to obtain a surface having a satin finish, or on its cathode side to obtain a surface having a mirror-like finish.
For the manufacture of printed circuit boards (PCB), a low surface roughness, i.e. a mirror-like surface finish, is preferred. Therefore, the thin functional foil is in practice always electrodeposited on the cathode side after deposition of the release layer. Typically the surface roughness is given by the Rz parameter, which is for the cathode side: 1.5 μm≦Rz≦3.5 μm. Since the thin functional foil surface conforms to that of the cathode side, its surface roughness is substantially equivalent to that of the cathode side.
Such a composite copper foil has proved very advantageous for the manufacture of PCB's. Indeed, during lamination of the composite copper foil on a resinous insulating substrate the thin functional foil is effectively protected by the carrier foil against resin bleed-trough and surface damages. Hence, after release of the carrier foil there is provided a copper clad laminate having a very smooth surface.
With regard to actual and future miniaturization requirements, it is desirable to increase the number of conducting elements per surface unit and to reduce the size of the conducting elements and the space between them (pitch). Such circuit patterns may only be obtained by applying very high density circuit pattern photo-definition and chemical etching processes to ultra-smooth and defect free surfaces. It will be appreciated that known composite copper foils do not provide a functional foil surface which is sufficiently free of defects for these increased miniaturization requirements. Indeed, cathode drums generally exhibit surface flaws which are responsible for the presence of striae on the cathode side of the carrier foil and consequently also on the surface of the thin functional foil formed thereon. Until now these striae have not been considered as problematic for the manufacture of circuit patterns, but this is no longer true when facing future miniaturization specifications.
One way to improve the surface quality of functional foil could be to form the cathode side of the carrier foil on a cathode drum having an ultra-smooth surface roughness, in particular free of surface flaws. But the actual technique does not allow the manufacture of ultra-smooth cathode drums at reasonable costs. Furthermore, such an ultra-smooth cathode drum would be easily damaged, and the maintenance of such an ultra-smooth cathode drum would be very expensive.